The present invention relates to a cooling system for cooling turbine bucket airfoils and particularly to a cooling system employing radial cooling passages for convectively cooling the airfoil and convectively and film cooling the trailing edge region of the airfoil.
Over the years gas turbines have trended towards increased inlet firing temperatures to improve output and engine efficiency. As the gas temperatures have increased, bucket airfoils at the higher metal temperatures exhibit substantial creep damage. The creep damage deteriorates until creep rupture takes place and damages the turbine flow path components. Moreover, with increasing firing temperatures, inlet temperature profiles have a tendency to be hotter at the tip of the airfoil as the hot combustion gas is centrifuged radially outwardly towards the tip. This renders the higher spans of the airfoils more susceptible to creep damage. Also, because of the shape of the airfoils, the trailing edges have increasingly exhibited distress including oxidation, creep and low cycle fatigue cracking as the airfoils see increasing temperatures.
In prior bucket designs, electrochemical machining (ECM) methods have been used to form cooling passages generally radially through the airfoil of the bucket. Particularly, cooling holes are formed using the shaped tube electrochemical machining (STEM) process by which holes are “drilled” using a round guide tube which electrochemically erodes a passage which is the same shape as the guide tube. The guide tube drilling extends the entire length of the airfoil. A separate drill pass is typically initiated at the bottom of the dovetail attachment of the bucket to the rotor wheel and meets the passage eroded by the airfoil STEM drilled passage. Thus, cooling air conveyed from the dovetail of the bucket to the tip of the airfoil through these drilled passages convectively cool the airfoil. However, the shaped tubes tend to wander as the length of the drilled hole increases. Also, the airfoils are shaped such that the trailing edges are very thin for aerodynamic efficiency. As a consequence, the STEM drilled holes can only be drilled within a certain distance from the trailing edge and necessarily leave a large portion of the trailing edge still relatively uncooled. Consequently, there is a need for a system for more effectively cooling trailing edge regions of a bucket airfoil.